This invention relates to power line signalling systems, and more particularly to apparatus for transmitting information from a transmitter to a receiver via an alternating current (AC) power line connecting the transmitter and receiver.
It is frequently desired to transmit information or data between locations which are connected to one another by an alternating current (AC) power line. For example, it may be desired to cause one or more remotely located devices, all of which are connected to an AC power line, to respond to commands from a central location, also connected to the power line, without the need for or cost of a separate communication channel (e.g., additional wires or a radio link) between the central location and the remotely located devices. The command signals may be address information to select one or more of plurality of the remotely located devices, instruction information to cause the remotely located device or devices to perform one or more of a plurality of operations, data to be processed by the remotely located device or devices, or the like. For convenience herein, the term data will be understood to include all such information.
In situations of the type described above, it is known to transmit information from the sending station to the receiving station via the AC power line connecting them. One prior art technique for accomplishing this is to superimpose on the AC power signal an information signal having frequency substantially higher than the power signal frequency (typically 50 or 60 Hz). Such higher frequency information signals may be unacceptably attenuated if required to travel too great a distance on a typical AC power line, and may also be blocked entirely by a power line transformer. It is also known to transmit information via a power line by clipping or suppressing the peaks of selected half cycles of the power signal, the clipped peaks being all of the same polarity. However, this causes the DC average level of the power signal to shift which may undesirably affect the operation of apparatus connected to the power line. In addition, clipping the peaks of the power signal may cause undesirable high frequency transients on the power line.
In view of the foregoing, it is an object of this invention to simplify and improve power line signalling apparatus.
It is a more particular object of this invention to provide a power line signalling system which does not employ easily attenuated high frequency information signals and which therefore permits transmission of information for greater distances over AC power lines.
It is another more particular object of this invention to provide a power line signalling system which does not cause a shift in the DC average level of the AC power signal and which substantially avoids high frequency transients on the AC power line.